It is well known that complex waveshapes of musical tones can be synthesized by combining a plurality of sine waves which are harmonic overtones of a fundamental sine wave. By varying the relative amplitudes of the different harmonic overtones, the tonal quality can be changed. In analog type synthesizers, time variant filters are used to change the tonal structure. Such filters are commonly called "sliding formants". The equivalent of sliding formant filters have also been incorporated in digital tone generators. In general this has required the individual harmonic coefficients to be controlled as a function of time. However, in some types of digital tone generators, such as that described in U.S. Pat. No. 3,315,792, the individual harmonic coefficients are not available in generating the musical tones. Rather, fixed waveshape data is stored in a read-only memory. Even where the harmonic coefficient data is available, modification of this data as a function of time and the calculations necessary to generate the resulting waveshape data can be a complex and time consuming operation.